In conjunction with a plurality of technical applications there is a need of an accurate dosage of pieces or parts of a plastic material. Such accurately dosed pieces of a plastic material is of importance in widely differing fields, e.g. in the plastics and the foodstuff processing technologies.
A fundamental difficulty in conjection with an accurate weight dosage of a plastically extruded material having a constant cross-section resides in the fact, that air inclusion in the plastic compound results in variations in the weight of the compound per unit of length. Another difficulty resides in the fact that many doughy compounds, e.g. doughs for pastry-work, to some degree are compressible. Consequently, variations as to feed pressure cause variations in the weight per unit of length of the extruded product.
Thus, a dosage, which is based on measuring the extruded length or volume, requires a feeding to the dosage device under a constant pressure and without risk of air inclusion formation in the extruded compound.
In the patent literature, there is a multitude of devices, which are designed for dosage of a plastic material or a doughy material and which are based on a volumetric portioning out of the flow of the compound. Examples of such devices are those described in the U.S. Pat. Nos. 4,332,538, 2,791,975 and 2,787,832. Said devices, which are based on a volumetric dosage, have a drawback, because pressure variations in conjunction with the feeding may result in dosage errors in case the plastic or doughy compound is compressible. Also, air inclusions results in dosage errors, which may be an important source of errors.
In the printed international publication WO 84/02633 and in U.S. Pat. No. 3,489,104 and the German patent specification No. 449,926 devices designed for volumetric dosage are described, which are provided with a roller feed. By means of the roller feed a comparatively constant pressure is attained in conjunction with the feeding to those parts of the devices, which perform the volume dosage. However, said devices are designed in such a way, that only a portion of each of the feed rollers is in contact with the plastic compound or the doughy compound in conjunction with the feeding of the compound between the rollers. This leads to the drawback, that seals are required, which engage the rollers and delimit that portion of the rollers being in contact with the compound. The necessary seals require a very accurate adjustment to complete eliminate a leakage of the plastic material and an air penetration. Also, the seal components are wear parts, which require regular servicing and regular replacements, which result in expenses.